The present invention relates to electronic oscillator circuits, and more particularly to complementary metal-oxide-semiconductor field-effect transistor oscillator circuits.
Many known complementary metal-oxide-semiconductor (CMOS) field-effect transistor oscillator circuits operate with voltage peaks occurring at internal nodes of the circuit that are beyond the voltage range taken by the circuit voltage supply. Such operating characteristics cause difficulties when these oscillator circuits are implemented in monolithic integrated circuit form. The first of these difficulties relates to the stability of the oscillation cycle, i.e. the duration of the oscillation period. When internal voltage values in the oscillation circuit exceed the power supply voltage value or go below the ground reference voltage, the pn semiconductor junctions surrounding the source and drain regions in the substrate of the p-channel and n-channel transistors, respectively, will be driven into being forward-biased. Because of this occurrence, and because of the well-known strong temperature dependence of such forward-biased junctions, the oscillation period will become quite temperature dependent because this period is altered by the changing switching characteristics of these pn semiconductor junctions. Also, the oscillation period duration will depend additionally on the variations occurring in the power supply voltage.
Another problem resulting when such internal circuit voltages occur outside the range taken by the circuit voltage supply is the problem of "latch-up." That is, various parasitic bipolar devices, such as transistors and silicon controlled rectifiers, are always present and can be activated by such voltage excursions. Particularly, silicon controlled rectifier action can occur along the surface of a CMOS monolithic integrated circuit chip causing the surface regions to go into a "latch-up" state preventing normal operation of the transistors.
Thus, a desirable CMOS oscillator will not have internal voltage range excursions which go beyond the range of the circuit voltage supply.